In response to the energy crisis and the greenhouse effect, wind energy has become the most cost-effective of all currently exploited renewable energy sources. In recent years, offshore wind energy has attracted more attention due to better wind conditions and negligible visual impact compared with onshore wind energy. With a large demand for offshore wind energy, wind turbine gets larger and larger for offshore application. The dynamic of a modern megawatt wind turbine is governed by the complex interaction of its subsystem and design requirements of multidisciplinary knowledge in various areas: rotor aerodynamics, control, mechanical systems, electrical systems and civil engineering. The design problem is particularly complicated because we need to predict the dynamic response under various conditions for the coupled system.This study is directed towards dynamic load calculation and response analysis to better understand of the structural dynamics of offshore wind turbine through mathematical model and analysis. The drivetrain is the most responsible for downtime of the wind turbine. To date, most of the state-in-art software or codes developed in wind energy industry adopt simplified drivetrain models so that no enough information is predicted from those models. Two different mathematical models are developed according to multibody dynamic theory. The proposed model accounts for the variation in the number of teeth in contact and support bearing elasticity, which are known to have influence on the dynamic behaviour of the drivetrain significantly. The torsional dynamic model is developed and verified. Then it is extended to a multibody dynamic model to include not only the translational vibration but also the torsional vibration. The vibrations of the critical components as well as the tooth contact force between different gear meshes are analyzed in detail.A lumped mass model for the offshore wind turbine tower and foundation is developed to investigate the dynamic response of the offshore wind turbine. Based on the distributed properties of the conical tower with circular section, the lumped mass properties can be obtained. This model is more efficient than the FEM model and will be extended to study the dynamic behaviours of other types of tower and foundation, such as jacket or dolphin structures.